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Caligus elongatus and Lepeophtheirus salmonis infestations of wild, return-migrant Atlantic salmon

Sea lice infestations of wild 1SW and 2SW salmon were monitored at Strathy Point, N Scotland. This is an interceptory fishery and the sampled fish are representative of a large proportion of the multiple river populations that comprise the ICES southern European stock component. L. salmonis (mean annual abundance 17-31 lice.fish-1) was numerically more abundant than C. elongatus (3-24 lice fish-1) in all except one of the seven monitored years. Prevalence of infestation was 100% for L. salmonis and 92-100% for C. elongatus. C. elongatus has been markedly under-estimated in its importance on wild salmon in the past and the present data are the most detailed available for wild fish. A positive association in abundance of the two species on individual fish leads to the deduction that fish are equally vulnerable to either species within any one year. Variation in annual abundance patterns was analysed for a range of parameters, with host condition factor being a likely candidate. Host condition was found to vary significantly among years but there were no negative correlations between lice abundances and host condition factor for either species. That is, increased parasite abundances do not determine (reduced) host condition factor, and poor condition fish are no more vulnerable to infestation than are high condition fish.

Reported by

University of St Andrews, Gatty Marine Laboratory
East Sands
KY16 8HL St Andrews, Fife
United Kingdom
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